What Is Horsepower?

What Is Horsepower?

Last Updated October 5, 2023 | Alison Smith

Horsepower is a common term used in the auto industry to describe how much power an engine produces. Although horsepower is an arbitrary unit of measurement, there is science behind it. So, what is horsepower exactly?

What Does Horsepower Mean?

Front of black steam engine on track

Horsepower, quite literally, is the power that one horse can produce. Of course, that doesn’t make much sense without some context.

Scottish engineer and inventor James Watt termed the word “horsepower” in 1782. Watt turned to horses as a way to potentially market his new steam engine. If he could prove that it was more efficient than a horse, that would be a major selling point.

Watt observed workhorses to try and determine how much power each produced. Even though all horses aren’t created equal, he estimated that one horse could turn a mill wheel with a 12-foot radius approximately 144 times an hour with a force of 180 pounds. From this, Watt was able to quantify horsepower.

How Much Horsepower Does a Horse Have?

One horse is capable of doing 33,000 foot-pounds of work in one minute. This means that a horse exerting one horsepower can lift a 330 lb. weight 100 feet in one minute, a 33 lb. weight 1,000 feet in one minute, or a 1,000 lb. weight 33 feet in one minute.

Horsepower diagram illustrating a horse pulling 33,000 pounds on a pulley

How to Calculate Horsepower

To calculate the horsepower of a vehicle, you need to know the engine speed and torque. You can find this information in your owner’s manual or by doing a Google search of your car’s make, year, and model. You’ll also need to know the formula for horsepower:

Horsepower = (RPM x Torque)/5,252

First, multiply your vehicle’s engine speed (RPM) and torque. Then, divide that number by 5,252, which is the RPM at which horsepower and torque are the same. The resulting number will be your vehicle’s horsepower. You can also convert horsepower into other units including joules and BTUs (British thermal units).

How Is Horsepower Measured?

Auto manufacturers need a standardized way of measuring horsepower. Since 1905, the Society of Automotive Engineers (SAE) has largely been in charge of those standards. The main way people measure horsepower is gross vs net.

Gross Horsepower vs Net Horsepower

For most of the 20th century, car engines were measured in gross horsepower. But in 1972, the SAE switched to net horsepower as a more accurate method of measurement. It should be noted that neither method takes into account the loss of power from the drivetrain. They’re just a measurement of engine power.

Gross Horsepower

Gross horsepower is the power output of an engine measured at the crankshaft or flywheel. Prior to 1972, automakers tested their engines on a dynamometer (dyno) and used SAE standards J245 and J1995 to rate the gross horsepower.

Dynamometer with engine testing horsepower

Engines were often fitted with headers instead of exhaust manifolds. Horsepower was measured without an exhaust system, air cleaner, or any accessories attached. Atmospheric conditions such as barometric pressure, humidity, and temperature were all set to maximize the horsepower numbers.

Because of the stripped down nature of the engines and manipulation of testing conditions, gross horsepower isn’t an accurate measurement of actual horsepower.

Net Horsepower

When SAE released new standards in 1972, the auto industry began using net horsepower instead of gross. Net horsepower is still tested on a dyno, just like gross horsepower. However, the testing parameters changed when SAE Standard J1349 was adopted.

In SAE Standard J1349, testing was still completed by having the engine on a test stand. New parameters required that the engine be tested in its factory-installed condition, which meant using the stock ignition timing, exhaust system, carburetion, and any accessories. This gave a more realistic approximation of the true power output of the engine once installed in the car.

Of course, horsepower numbers decreased across the board once the loss of power was factored in. While it seemed like many cars were dropping in power, that wasn’t the case. It was simply that the rating for horsepower changed.

The J1349 rating was used until 2005, when SAE began using the J2723 rating. This standard created SAE Certified horsepower ratings for manufacturers.

The SAE Certified horsepower standard is voluntary and not used by all carmakers. It is nearly identical to standard J1349, with one main difference. Other than clarifying the terms, the new standard requires a third-party witness as its main provision. For a carmaker to boast their horsepower as "Certified to SAE J1349®,” they must have an SAE-qualified witness observe the testing process to assure all measurements are conforming to the J1349 standards.

The Future of Horsepower

Although car makers have measured horsepower in pretty much the same way since 1972, that could change with the growing popularity of electric vehicles. As you may know, electricity is measured in watts. Coincidentally, watts got their name from horsepower inventor James Watt.

So what does that mean for horsepower moving forward? Luckily, watts can be converted into horsepower.

How Many Watts are in a Horsepower?

One horsepower equals 745.7 watts, often rounded up to 746. To figure out how much horsepower an electric car produces, simply divide the number of watts by 746. Using the 2019 Tesla Model S as an example, it’s 193,000-watt (193 kW) electric engine produces the equivalent of 259 horsepower.

Since you can convert the wattage of an electric motor into horsepower, it’s safe to say that horsepower isn’t going anywhere anytime soon.

Sources: Why Do We Measure Engines Against the Power of Horses?, Chevy Hardcore | About SAE J1349 Certified Power, SAE | How Horsepower Came to Be, Popular Mechanics | Horsepower vs Torque: What's the Difference?, Car and Driver | How to Calculate Horsepower, WikiHow

Image Credit: Creative Commons

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